Geoscience Reference
In-Depth Information
3.1
Introduction
The Indo-Gangetic Plains (IGP), one of the most agriculturally fertile belts of the
world, are home to ~900 million people living in Pakistan, northern India, Nepal,
and Bangladesh. These plains are fed by the Indus and Ganges rivers, i.e., down-
stream of the towering Himalaya in the north. Rapid industrialization and urbaniza-
tion have posed environmental concerns to this densely populated region in recent
years. The air quality and climate over the IGP has also been affected due to increas-
ing concentrations of anthropogenic aerosols (Massie et al.
2004
; Ramanathan and
Ramana
2005
; Gautam et al.
2007
; Hsu et al.
2012
; Kaskaoutis et al.
2012
). During
the winter season, the IGP suffer from western disturbances (a series of alternate
low and high pressure systems), which move from west to east, leading to intense
haze and fog in the region. A low pressure system results in enhanced moisture
content in the boundary layer, high winds, and clouds, which is subsequently
replaced by a high pressure system leading to clear sky conditions, low winds, radi-
ative cooling of the ground, and temperature inversions (Pasricha et al.
2003
).
During this period, the temperature also reaches its annual minima with increased
frequency of western disturbances. These conditions are ideal for the accumulation
of pollutants within the boundary layer and often result in fog and haze formation
over the IGP (Hameed et al.
2000
; Gautam et al.
2007
).
The widespread fog, clearly visible in satellite images, extends over an area of
~1,500 km in length and ~400 km in width, with severe fog events blanketing the
entire IGP including parts of Pakistan and Bangladesh on the western and eastern
sides (see Fig.
3.1
), respectively. Majority of the fog episodes last from mid-
December to mid-January causing poor visibility which results in major disruptions
to the air and rail transportations as well as signifi cant number of deaths due to
vehicular accidents (Hameed et al.
2000
). The number of foggy days during winter
has been increasing in recent years compared to earlier decades (Singh et al.
2004
;
Jenamani
2007
) with strong increasing trends of anthropogenic pollution in the IGP
(Habib et al.
2006
; Sarkar et al.
2006
). In addition, trends in poor visibility days due
to haze and fog during winter season have been signifi cantly increasing over the
IGP amounting to 90 %, i.e., almost everyday (De and Dandekar
2001
).
At the outset, the occurrence of fog is not unique to this part of the world and
certainly not a recent phenomenon over India. However, the intensity, the persistence,
and the widespread nature of the winter fog are overwhelming. Given the exorbitant
impact on public life, it has almost become equivalent to a natural hazard which is
unlike on any other part of the world. In simple terms, fog is a cloud near the surface
with visibility less than 1,000 m. It requires the suffi cient amount of moisture in the
air and cold temperatures together with frequent pressure variations - collectively
and commonly known as cold waves over the IGP, during winter season.
In the past three decades, along with the population growth, atmospheric pollu-
tion has risen alarmingly over the IGP due to increase in various emission sources.
Some of the increasing anthropogenic sources include the density of coal-fi red
power plants, associated with growing energy demand, and number of vehicles of
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